This invention relates to an autotensioner for keeping the tension in a belt such as a camshaft-driving belt in a vehicle engine at a constant level, and a belt tension adjustor using such an autotensioner.
FIG. 16 shows a belt transmission system in an internal combustion engine for driving engine camshafts. It comprises a pulley P1 mounted on an engine crankshaft 1, pulleys P2 mounted on camshafts 2, a pulley P3 mounted on an oil pump drive shaft 3, and a timing belt 4 trained around these pulleys to transmit the rotation of the crankshaft 1 to the camshafts 2 and the drive shaft 3.
The belt 4 tends to stretch with time due to aging and the distances between the pulleys change due to thermal expansion of the engine during operation. The tension in the timing belt 4 thus changes. In order to absorb such change in the belt tension, a tension adjustor is used. The tension adjustor of FIG. 16 comprises a pulley arm 6 pivotable about a shaft 5 and carrying a tension pulley 9, and an autotensioner 7 including an outwardly biased pushrod 8 to press the pulley 9 against the belt 4. As the tension in the belt increases or decreases, the pushrod 8 is pushed into or protrudes from the cylinder of the autotensioner 7 to absorb any change in the belt tension and keep the tension constant
Such autotensioners are disclosed e.g. in Japanese patent 1891868 and Japanese patent publication 7-117130.
As the timing belt stretches gradually due to aging, the pushrod 8 protrudes from the cylinder to absorb any change in the belt tension.
For a belt transmission system having no such autotensioner 7, it is recommended to replace the timing belt 4 when the vehicle has traveled 100 thousand kilometers. But by using an autotensioner, it is possible to make the belt tension stable and prevent flapping of the belt and thus to increase its durability. This makes it possible to prolong the guarantee period of the timing belt even until the odometer reading far exceeds 100 thousand kilometers.
But the stroke of the pushrod 8 is limited. When the pushrod advances to a position exceeding its effective stroke, the autotensioner will lose its tension adjusting capability, i.e. the ability to keep constant the tension in the belt. Thus, the belt becomes slack and begins to flap. This increases the possibility of xe2x80x9cjump of pulley teethxe2x80x9d and decreases the durability of the belt. In the worst case, the belt may be damaged.
An object of the invention is therefore to provide an autotensioner and a tension adjustor having a means for detecting the belt replacement timing.
According to the invention, there is provided an autotensioner comprising a cylinder, a pushrod slidably received in the cylinder, and a spring for biasing the pushrod so as to protrude out of the cylinder, the pushrod being movable in an axial direction to absorb any change in the tension in a belt, characterised in that a detector for detecting a limit position of a forward stroke of the pushrod is provided.
With this arrangement, when the belt has stretched with time due to aging, the pushrod moves outwardly. The detector means detects the fact that the pushrod has moved to the limit position. The signal from the detector informs that the autotensioner is malfunctioning and that the time to replace the belt has come.
The detector means may comprise an electrode mounted to the cylinder through an insulating member, and a contactor provided on the pushrod so as to come into contact with the electrode when the pushrod has moved to the limit position.
In the abovesaid arrangement, the electrode may be a metal ring for reinforcing an oil seal closing an opening of the cylinder, and the contactor may be a guide flange fixedly mounted on the pushrod and guided along the inner periphery of the cylinder so as to contact the exposed portion of the metal ring to detect the limit position.
The pushrod may be formed with a small-diameter portion at its upper portion and the contactor may be a shoulder formed at the root of the small-diameter portion, or a pin mounted on the pushrod near its tip to extend radially outwardly. The electrode is provided at such a position that the shoulder comes into contact with the electrode when the pushrod has moved to the limit position.
The detector means may comprise a detecting coil mounted on the inner wall of the cylinder so as to be coaxial with the pushrod, a detection piece provided on the pushrod near its tip and having a different magnetic permeability from the pushrod, the detection piece having a bottom edge defining a boundary between the piece and the pushrod, the bottom edge being located so as to oppose the coil when the pushrod has moved to the limit position, whereby the limit position is detected based on a change in output of the detecting coil.
The detector means may comprise a ring-shaped permanent magnet integral with the pushrod, and a sensor mounted on the outer surface of the cylinder for detecting the magnetic force produced from the permanent magnet, the permanent magnet being located so as to oppose the sensor when the pushrod has moved to the limit position.
The detector means may comprise an additional sensor for detecting the magnetic force produced from the permanent magnet, the additional sensor being mounted at such a position that the permanent magnet opposes the additional sensor before the pushrod moves to the limit position.
As the sensor for detecting the limit position of the pushrod, a Hall sensor with an analog output may be used. Since the analog output from the Hall sensor changes with the movement of the pushrod and thus the permanent magnet, the position of the pushrod can be detected continuously by converting the analog signal from the Hall sensor to a digital signal by means of an A/D converter and giving the digital signal to a CPU of the control unit for computation.
In detecting the position of the pushrod, the magnetic force of the permanent magnet decreases as the temperature increases whereas the Hall sensor has an output offset with temperature change. Thus some error can occur in the detection of the position of the pushrod with temperature change.
The detector means may further comprise a temperature sensor and a temperature compensation unit for correcting the output of the sensor based on the output from the temperature sensor.
The temperature sensor may be provided near the sensor.
The temperature sensor may be one originally provided in a vehicle on which is mounted the autotensioner.
According to this invention, there is also provided a belt tension adjustor comprising a pivotable pulley arm carrying a tension pulley for tensioning a belt, and an autotensioner comprising a cylinder and a pushrod structured to be biased toward the pulley arm to pivot the pulley arm in such a direction that the tension pulley is pressed against the belt, characterized in that a switch is provided at such a position that when the pushrod has moved to a limit position of its forward stroke, the pulley arm abuts and activates the switch.
In the belt tension adjustor, when the pushrod moves outwardly until it reaches the limit position, the pulley arm will actuate the detection switch, thus notifying that the autotensioner is malfunctioning.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: